It is well known to use thermoelectric modules for various cooling devices, e.g. portable refrigeration boxes and for active cooling of integrated circuits like processors. It is also well known to use thermoelectric modules for power generation. If designed and sized properly, exhaust gas- or recycled exhaust gas (EGR) driven thermoelectric modules used as generators may reduce the fuel consumption and hence the CO2-emissions from combustion engine powered machinery and vehicles.
A thermo electric generator system powered by hot exhaust gases consists of inlet and outlet connections to the exhaust gas or EGR streams, a first and a second heat exchanger surface and a thermo electric module arranged between and in contact with the first and the second heat exchanger surfaces. The first heat exchanger surface provides heat transfer from the hot exhaust gas. The second heat exchanger surface is typically connected to an external cooling system.
The thermoelectric module contains at least one thermoelectric element that through a temperature difference between its surfaces convert heat into electrical energy. The generated electrical energy may be handled by power electronics and distributed to power consumers around the combustion engine system, or the machinery that is powered by the combustion engine. Accordingly, the heat energy that normally is expelled through the exhaust gases into the ambient air may be recovered to some degree.
Common for these thermoelectric modules is that they require adequate surface contact with the first and second heat exchanger surfaces. In practice this may be done by applying an axial force or pressure on the core of heat exchanger plates, in which core thermoelectric modules are arranged intermediate the heat exchanger plates. The axial force or pressure may be applied by applying the force from the outside on two opposing rigid end plates, see e.g. DE102007063196A1. Yet another example disclosing a radial compression is disclosed in DE102005005077A1. It is also known to apply an axial pressure within the core. This is by way of example disclosed in AT506262A2, in which a cold tube forming one layer in the core is compressed during assembling of the core, thereby exerting an axial force pressing the thermoelectric modules towards the adjacent warm tubes.
There are several problems to overcome in the design of this kind of assemblies. The performance normally increases with higher temperature differences over the surfaces of the thermoelectric module, meaning that the heat flow through the heat exchanger should be as effective as possible, while at the same time the heat transfer through the thermoelectric module should be as low as possible to maintain the temperature difference.
The contact surface and the contact pressure between the thermoelectric module and the heat exchanger surfaces are important factors to provide a good heat transfer. Therefore, the thermoelectric module is designed by the supplier thereof with a dimensioning contact pressure to provide an optimal operation.
A poor contact or a gap between the thermoelectric module and the hot side of the heat exchanger plate will cause the hot side of the thermoelectric module to be colder, whereby the temperature difference over the thermoelectric module will decrease and hence also electrical performance.
Vice verse, a poor contact between the thermoelectric module and the cold side of the heat exchanger surface will cause the cold side of the thermoelectric module to be hotter, whereby the temperature difference over the thermoelectric module will decrease and hence also electrical performance. In this case there is also a risk that that the thermoelectric module becomes overheated depending on the temperature of the hot fluid.
It is also important that the pressure is uniform across the surface of the thermoelectric module.